Lesson: Multi-Threading

Overview: 
Explore the concepts and details of using more than one Thread of execution in a program.
Objectives: 

Understand what Processes and Threads are and what it means to have multiple Threads active in Java program. Gain a basic understanding of how to use Threads.

 

Content: 

An executing Java program is called a Process. That process executes your instructions (program) sequentially following the path you created when you wrote your program. This single path is called a thread. As such, your program is only doing one activity (Java statement) at a time and working on one task (your programs list of statements) at a time. For most situations, this works fine. But there are times when you would like to have your program doing more than one thing at time. Using Java Threads it is possible to create additional threads or paths of execution that run in parallel with the main (or first) thread. The effect is that your program can be doing more than one thing (task) at a time. Doing robotics on our platforms you will not need additional threads (multi-threading) most of the time but there are some cases where multi-threading can be useful. Threads can be used to simplify the main path in your program or to perform repetitive tasks while the main thread is busy with something else, sleeping or waiting for some user action.

Like many aspects of programming in Java (and other languages), threads are simple in concept but potentially complex in implementation. There are several ways to do threading and multi-threading can create very interesting and hard to find bugs in your program. But it is possible to keep it simple and get benefit from multi-threading without getting into too much of the possible complexity. We are going to explore basic threading here and there will be example programs in each of the platform sections. It is probably best for you to go now to the section for your hardware platform and work through the examples until you come to the one on using Threads. Then return here to finish this lesson.

When creating a new thread of execution in a Java program, there are two ways to do it. You can use the runnable interface or you can extend the Thread class. We are only going to look at extending the Thread class.

When creating a new thread, the main thing we need to define is the code you want executed in that thread. When you create a new thread, you are telling the JVM here is a bit of code, start running it separately from the main thread and run it until the code path comes to an end. The thread code can have it's own private variables that exist only while the thread is running and the thread shares the class level variables of the class that creates the thread, assuming the thread class is an inner class. An inner class is a class within a class and doing threading with inner classes greatly simplifies things.

Lets look at a simple example:

Here the main thread prints the value of i every half second and the thread increments the value every second. You can see by the results of this code that the two threads run independently of each other.

The code we want to run in the thread is put in the run() method of the thread class. Threads are started with the start() method and stopped with the interrupt() method. When thread.interrupt() is called, the Thread class isInterrupted() method returns false. You should look for this to exit your thread code. If you happen to be in a blocking method when interrupted, sleep() in this case, the InterruptedException will be thrown. You normally just ignore that exception as it is just another signal to stop your thread code. The second catch statement catches and reports any errors that might occur in your code.

Here is this example in CodingPoint. Threads can be used in many ways and there are many methods on the Thread class for managing and coordinating threads. More complex threading is beyond the scope of this lesson and if you use threads it is best to keep it simple as shown here.

When doing multiple threads, you frequently need to share data between threads. Coordinating access to shared variables is called concurrency and is a complex and multi-faceted subject. Java contains many features to allow threads to coordinate write access to shared variables and objects by multiple thread. Again these features are beyond the scope of this lesson. To keep it simple and avoid concurrency issues, follow this rule: variables should only be updated by one thread. They can be read by several threads but only changed by one. In the above example, only the MyThread class changes the value of variable i.

Here is a simple tutorial on multi-threading, here is a more detailed tutorial including concurrency and here is the official Java documentation on threading.

 

 

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